This invention relates to optical devices, and in particular to providing a proper coating to the devices for improved performance.
Dielectric coatings play an important role in achieving desired performance of optical devices. For example, an anti-reflection coating on the facets of a III-V or II-VI semiconductor laser or on the surface of a light-emitting device or photodetector can increase the quantum efficiency of the device. In the case of a distributed feedback laser, the anti-reflection coating can also produce single longitudinal mode emission by suppressing the Fabry-Perot modes. A passivation coating on the facets of a III-V or II-VI laser or light-emitting diode can prevent oxidation at the semiconductor surface or evaporation of the elements of the device, thus permitting higher device reliability.
A variety of materials has been proposed for such layers including ZrO.sub.2, Al.sub.2 O.sub.3, SiO.sub.2 and SiN.sub.x. The SiO.sub.2 and SiN.sub.x layers are usually deposited by sputtering, which can cause damage to the semiconductor surface. Electron-beam deposition of coatings such as ZrO.sub.2 should not cause significant damage, but the addition of oxygen is usually required in the deposition process to get the proper stoichiometry for a desired refractive index. This requirement makes it difficult to form the layer reproducibly. While Al.sub.2 O.sub.3 can be easily e-beam deposited, the values of reflectivity of coated laser facets with such layers tend to be high (greater than 4 percent) and, again, addition of oxygen is usually required to get the desired refractive index.
It is, therefore, an object of the invention to provide a passivation or anti-reflection coating for III-V and II-VI semiconductor devices with high yield and without adversely affecting device performance.